1. Field of the Invention
The present invention relates to a transmitter having a high power amplifier (HPA), and more particularly to a predistortion digital linearizer for the transmitter.
2. Background of the Related Art
A high power amplifier amplifies a radio frequency (RF) signal from a base station of a communication system to the air. The high power amplifier thus can have much influence on the non-linearity of the entire system.
In order to improve the linearity of the HPA, various techniques are used. These techniques include a feed forward method, an envelope feedback method, and a predistortion method.
The predistortion method has been widely adopted as a linearizing method due to the linearizer's low price compared with its performance and its operation characteristic operable in a broader band width.
In the predistortion method, an input signal is initially distorted in a manner opposite to the nonlinear distortion characteristic of the power amplifier. The predistorted signal is then inputted to the high power amplifier, resulting in an output signal having improved linearity.
FIG. 1 is a schematic view showing the construction of a related art predistortion linearizer in an analog form. As shown in FIG. 1, the predistortion linearizer includes a first directional coupler 1 detecting a sampling signal from an input signal and a phase shifter 2 shifting a phase of the input signal outputted from the first directional coupler 1. It further includes a variable attenuator 3 varying the magnitude of the input signal outputted from the phase shifter 2, a high power amplifier (HPA) 4 amplifying the power of the input signal outputted from the variable attenuator 3, and a second directional coupler 5 for sampling a signal for comparing the input signal from the output of the HPA 4. Comparators 6 and 7 are also provided for comparing the signals respectively outputted from the first and the second directional couplers 1 and 5 and controlling the phase shifter 2 and the variable attenuator 3 by using the comparison resulting value.
The operation of the related art analog type predistortion linearizer of FIG. 1 will now be described.
The first directional coupler 1 samples an input signal and the second directional coupler 5 samples an output signal of the HPA 4. At this time, a sampling ratio should be adjusted so that the two sampling signals have the same magnitude. Since the output signal of the HPA 4 is a power-amplified signal, it should be sampled to fit the magnitude of the input signal.
The sampled signals are inputted to each comparator 6, 7 by the first and the second directional couplers 1 and 5.
The comparators 6 and 7 compare the two inputted signals to obtain a signal difference, that is, an error value. The comparators control the phase shifter 2 and the variable attenuator 2 with the obtained error value, thereby performing predistortion for the input signal of the HPA 4.
However, the related analog type predistortion linearizer has various problems.
For example, since it is susceptible to a temperature change and influenced by noise, the accuracy of the predistortion is degraded.
In addition, since circuits for distorting the input signal should be controlled every time a signal is inputted, it cannot handle an input signal with a broader bandwidth, such as those used in the IMT-2000 (International Mobile Telecommunications-2000).
A gain-based perdistortor has previously been proposed to solve such defect. Such a predistorter, however, is actually implemented with an analog circuit, so that a circuit for adjusting a substantially obtained predistortion control value is relatively complicate compared with the digital method. Thus, supplementary circuits such as a temperature compensation circuit are additionally required.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative detail, features and/or technical background.